1. Field of the Invention
The present invention relates to a battery including a positive electrode, a negative electrode and an electrolyte.
2. Description of the Related Art
With progress in science and technology such as semiconductor technology, personal computers (PCs), mobile phones, and the like have been developed, and batteries utilized as power supply for electronic apparatus have been expected to facilitate handling of the batteries such as reduction in size and weight, and to exhibit excellent electric characteristics.
In view of such expectations, the following types of lithium-ion secondary batteries have been developed and have widely been used: a lithium-ion secondary battery having a graphite material that uses an intercalation reaction of lithium (Li) between graphite layers, or a lithium-ion secondary battery utilizing a carbonaceous material as a negative electrode active material to which lithium insertion and extraction into pores are applied.
In recent years, as increasing in power and time consumed for the electronic apparatus due to higher performance of the electronic apparatus, an increase in capacity and power generation of a secondary battery have been desired, and especially in capacity, an improvement in characteristics has much been desired. In particular, of battery capacity and cycle characteristics, more improvements of characteristics are desired and studies on arrangements of a negative electrode (current collector and active material layer formed on the surface of the current collector) have proceeded.
A battery including a negative electrode active material layer formed of a carbon material such as graphite is difficult to improve characteristics greatly because a battery capacity of the negative electrode active material is near the theoretical limit, and hence a research of an alloy-based negative active material containing silicon (Si) and tin (Sn) also has proceeded.
However, according to such alloy-based negative active material, a volume of a battery changes considerably when the battery is charged and discharged. As a result, large stress is applied to electrodes so that the electrodes are deformed, causing an interface between the positive and negative electrodes unstable or the structure of the active material layer collapsed, thereby resulting in cycle characteristics being lowered.
Also, as a method of improving cycle characteristics of the active material of which volume changes considerably, Japanese Unexamined Patent Publication No. 2003-7305 puts forward experiments on improving cycle characteristics by using an arrangement in which both principal planes of a thin plate-shaped current collector is formed as rough surfaces or an arrangement in which an active material is deposited on a current collector so that a current collector interface is alloyed with heat generated upon film deposition or subsequent heat treatment to thereby increase bonding strength have been made so far.
However, since the electrode formed by the above-mentioned method that applies large stress to the current collector, the electrode is caused to either crack or deform so that satisfactory characteristics may not be obtained.
Further, if it is intended to improve battery capacity by a negative electrode arrangement in which an active material layer is formed (supported) not on one principal plane but on both principal planes of a film-shape (strip-shape or thin plate-shaped) current collector, the electrode in which active material layers whose volumes change considerably are supported on both surfaces of the current collector is caused to apply large stress to the current collector when the battery is charged and discharged. As a result, it is unavoidable that cycle characteristics are lowered and that the electrode is either cracked or fractured.